McPartland, John M. 1997. Cannabis as repellent
and pesticide. Journal of the International Hemp
Association 4(2): 87-92 Cannabis has been used as a pest
repellent and pesticide in a variety of formulations. It has been planted as a companion crop to deter insects,
nematodes, fungi, and weedy plants. Dried leaves and flowers have repelled or killed insects, mites, nematodes, and
weeds. Plant extracts (either aqueous or polar organic solvent extracts) have killed or repelled insects, mites,
nematodes, fungi, weedy plants, bacteria, and protozoans. Pure cannabinoids reportedly inhibit or kill bacteria, fungi, and
insects. The validity of some of these reports is debated. Most of the scientific literature describes in vitro experiments,
few studies concern field work. Utilizing left-over Cannabis leaves against pests appears to be a possible use
for this harvest residue.

IntroductionCertain plants with repellent and pesticidal
properties have long been used to rid our crops, homes, and bodies of various pests (Grainge and Ahmed 1988). Famous
examples include garlic (Allium sativum L.), castor (Ricinus communis L.), marigold (Tagetes patula L.), tansy
(Tanacetum vulgare L.), neem (Azadirachta indica L.),
pyrethrum (Chrysanthemum cinerariifolium (Trevir.) Vis.), tobacco (Nicotiana tabacum L.), and strychnine
(Strychnos nux-vomica L.). Cannabis has also been used
as a pest repellent and pesticide. This paper documents these uses from the scientific literature.

Materials and methodsThe following data bases were searched with the
keywords Cannabis, hemp, marijuana, cannabinoids: AGRICOLA (1990-1996), Biological and Agricultural
Index (1964-1990), Review of Agricultural Entomology
(1913-1996, a continuation of Review of Applied
Entomology, Series A), Review of Plant Pathology (1922-1996, a continuation of Review of Applied Mycology),
MEDLINE (1984-1994), and Index Medicus (1964-1994).Cataloged holdings at the following libraries were
searched for texts concerning allelopathy, biopesticides, botanical pesticides, and biological control: Dartmouth,
Harvard, Michigan State University, National Agriculture Library, National Library of Medicine, Pennsylvania State
University, Stanford University, University of Illinois, University of Missouri, University of Michigan,
University of Pennsylvania, University of Vermont. All information regarding Cannabis was scanned for supporting citations,
and antecedent sources were retrieved.

ResultsCannabis has been utilized as a
pest repellent or pesticide, in a variety of formulations. These formulations include dried plant parts, plant extracts or pure
cannabinoids, as well as use of the genus as a "companion plant".

Companion plantsCompanion plants constitute a form of biological
control - the use of living organisms to manage unwanted pests and disease organisms. Cannabis plants have
been grown as companion plants alongside crops which require this protection. Riley (1885) noted that
Cannabis sativa growing near cotton exerted a "protective
influence" against cotton worms (Alabama argillacea, then called Aletia xylina). Similarly, hemp grown around
vegetable fields safeguarded the fields from attack by a cabbage caterpillar, Pieris brassicae (Beling 1932); potato
fields were protected against the potato beetle, Leptinotarsa decemlineata (Stratii 1976); wheat suffered
less damage by the root maggot, Delia coarctata
(Pakhomov and Potushanskii 1977); and root exudates of Cannabis repelled underground larvae of the European
chafer Melolontha melolontha (Mateeva 1995). Some of these reports have been refuted in subsequent studies
(Ziarkiewicz and Anasiewicz 1961, Mackiewicz 1962, Kurilov and Kukhta 1977).
Cannabis suppresses the growth of neighboring plants, whether they are noxious chickweed,
Stellaria media (Stupnicka-Rodzynkiewicz 1970) or valuable crops
such as lupine, beets, brassicas (Good 1953) and maize (Pandey and Mishra 1982). Hemp has been interplanted
with potatoes to deter the potato blight fungus, Phytophthora infestans (Israel 1981).Hemp has been rotated with potatoes to suppress
the potato cyst nematode, Heterodera rostochiensis
(Kir’yanova and Krall 1971). Hemp rotations also
suppressed soil populations of the root knot nematode, Meloidogyne chitwoodi (Kok et al. 1994). Some cultivars
of Cannabis are resistant to Meloidogyne hapla (de Meijer 1993). Scheifele et al. (1997) assessed the soil populations
of several nematodes, before and after a hemp crop (using cultivars ‘Unico B’ and ‘Kompolti’) in Ontario, Canada.
The hemp crop suppressed soybean cyst nematodes (Heterodera glycines), but increased the populations of
spiral nematodes (Heliocotylenchus or Scutellonema
species) and root knot nematodes (Meloidogyne incognita).Mateeva (1995) studied an unspecified
Meloidogyne on four different crops growing in Bulgarian
soil. After 30 days, cucumber plants averaged 56 root knots per plant and 396 Meloidoygne larvae were found in
the surrounding soil. Tomato plants averaged 42 root knots and 318 larvae, Cannabis plants averaged 5 root
knots and 21 larvae, and marigolds averaged 1 root knot and no larvae. Mateeva concluded, "by including
unfriendly plants in the rotation scheme with tomato and cucumber, it is possible to obtain a soil completely cleared
from root knot nematodes."

Plant extractsPlant extracts are a popular pesticide formulation.
Extracts are produced by soaking fresh or dried plant material in a solvent. The plant material may be mashed or
soaked whole. After an appropriate period of time, the solid material is filtered out, leaving the liquid extract. Aqueous
extracts are the most popular. Polar organic solvents, such as alcohol or ether, are sometimes used to extract lipids
from plant material.Many reports describing the use of
Cannabis extracts have omitted important information. For instance,
the plant part harvested, and when it was harvested (what stage of the life cycle)? How much plant material (by
weight) was soaked in the solvent (by volume)? How long was the material extracted, and at what temperature?

InsectsInadequately described extracts have killed insect
pests (Bouquet 1950, Abrol and Chopra 1963) and the mite Tetranychus urticae (Fenili and Pegazzano 1974).
Metzger and Grant (1932) sprayed Japanese beetles with a United States Pharmacopoeia (U.S.P.) 80% ethanol
extract, diluted to 1/64 strength. The spray weakly repelled adult beetles. (U.S.P. extracts are no longer
available.)Stratii (1976) boiled flowering hemp plants in
water and sprayed the decoction on potato plants to kill the potato beetle (Leptinotarsa decemlineata). Jalees
et al. (1993) killed mosquito larvae (Anopheles and Culex
species) with an ethanol extract. Bajpai and Sharma (1992) sprayed a 20% w/v cold water extract of "bhang"
on crop plants to reduce egg laying by Chilo partellus, a lepidopteran borer. A 20% w/v petroleum ether extract
killed 40% of the borers, and this toxicity persisted for four days.

NematodesMojumder et al. (1989) ground up 100 g of fresh
Cannabis leaves in 25 ml water and filtered it through
muslin cloth. This extract killed J2-stage juvenile nematodes (Heterodera cajani) in 6 hours. A 5% solution of the
extract also killed nematodes after 24 hours. Haseeb et al.
(1978) macerated 10 g of either roots or above-ground shoots in a Waring blender for 10 seconds and extracted
the mash for 24 hours in 75 ml distilled water. The shoot extract did slightly better than the root extract, killing a
variety of plant-pathogenic nematodes (Hoplolaimus
indicus, Rotylenchulus reniformis, Tylenchorynchus brassi-cae).
When diluted to 10% of its original strength, the root extract caused greater mortality than the shoot extract.

FungiFerenczy et al. (1958) extracted an Italian hemp
(cultivar ‘Bologniensis’) in "organic solvents and alkali". The extract did not inhibit in vitro growth of yeasts
(Saccharomyces, Rodotorula, Hansenula spp.) or
filamentous fungi (Aspergillus, Penicillium spp.). Ethanol extracts of leaves inhibited spore germination of Ustilago species
(Misra and Dixit 1979, Singh and Pathak 1984) and Neovossia indica (Gupta and Singh 1983).Upandhyaya and Gupta (1989) inhibited
Curvularia growth on petri plates with an aqueous extract,
prepared by extracting 10 g of Cannabis leaves in 100 ml cold water. Greater inhibition was achieved with an
ethanol extract, mixing 5 g leaves in 100 ml of 80% ethanol. In a similar experiment, Kaushal and Paul (1989)
inhibited growth of Colletotrichum truncatum, but not Septoria glycines or Ascochyta phaseolorum. Neither
aqueous nor ethanol extracts inhibited growth of the human pathogen Trichophyton rubrum or the opportunistic
pathogen Aspergillus niger (Gupta and Banerjee 1972).Pandey (1982) crushed 10 g of leaves in 100 ml
water and filtered the mash with filter paper. A 40% solution of the extract inhibited 25 different species of fungi
that infested stored seeds of finger millet (Eleusine cora-cana
L.), including species of Aspergillus, Penicillium,
Cladosporium, Drechslera, Fusarium, Cephalosporium, Rhizopus, Mucor and Curvularia. An aqueous extract of
hemp protected pine seedlings from a disease caused by an unnamed Fusarium species (Vysots’kyi 1962).

ProtozoansNok et al. (1994) prepared a seed extract by
crushing 10 g of dried seeds, mixing the powder in 200 ml petroleum ether, stirring continuously for 1 hour, filtering,
and then washing the filtrate with 100 ml portions of N/10 NaOH. The ether extract had a profound trypanocidal
effect on Trypanosoma brucei tested in vitro. Mice infected by T. brucei were injected with the extract (50 mg/kg/d)
and cured in 5 days.

Pure cannabinoidsCannabinoids are a family of C 21 terpenophenolic compounds uniquely produced by Cannabis (Turner
et al. 1980). Some studies reportedly working with pure cannabinoids are probably erroneous, such as those
working with aqueous extracts (Bajpai and Sharma 1992). Aqueous extracts should contain little cannabinoid, as
these compounds are not very water-soluble. In a study using ethanol extracts, Veliky and Latta (1974) located
two zones of bacterial inhibition at R f 0.87 and R f 0.61. These R f values
did not correspond to the tetrahydrocannabinol (THC) and cannabidiol (CBD) values they had
determined in a previous study (Veliky and Genest 1972). Nok et al. (1994) assumed that the ether extract they
successfully tested against Trypanosoma brucei contained cannabinoids. But they had extracted Cannabis seeds,
which do not contain cannabinoids except sporadically as contaminants from bract debris clinging to the outside of
the seeds (Máthé and Bócsa 1995).

FungiFlowering tops, where cannabinoid concentrations
are highest, are less susceptible to some fungal pathogens (Charles and Jenkins 1914, McPartland 1983). McPartland
(1984) extracted flowering tops with petroleum ether, then eluted the extract into its separate components with thin-layer
chromatography (using silica gel on glass plates). He sprayed eluted plates with spores of
Phomopsis ganjae suspended in a nutrient solution. Spore germination was inhibited by three components of the extract, located by
thin-layer chromatography at R f 0,
R f 0.20
and R f 0.68. The latter two zones consisted of CBD and THC, respectively.
Dahiya and Jain (1977) extracted THC and CBD
using adsorption column chromatography, (affirming correct identification with thin-layer chromatography and gas
chromatography), and assayed effects of these cannabinoids against in vitro growth of 18 fungi. Generally, THC
inhibited human pathogens (e.g., Microsporium and Trichophyton species) more than CBD. Conversely, CBD
inhibited plant pathogens (e.g., Alternaria alternata,
Curvularia lunata, Fusarium solani, Trichothecium rose-um)
more than THC. Two fungi were completely resistant to THC and CBD - Aspergillus niger and
Penicillium chrysogenum. Interestingly, these two species are
frequently isolated from moldy marijuana (McPartland and Pruitt 1997).

InsectsRothschild et al. (1977) conducted fascinating
experiments with tiger moths (Arctia caja). Tiger moth caterpillars, like monarch butterfly caterpillars, feed on
poisonous plants and store plant poisons in their exoskeleton. The stored poisons repel caterpillar predators,
providing an evolutionary advantage. Rothschild et al. (1977) fed the caterpillars a Mexican plant rich in THC. The
caterpillars stored some of the THC in their exoskeleton. But caterpillars fed a pure diet of high-THC plants were
stunted and did not survive beyond the third instar. Nevertheless, Tiger moth caterpillars preferred eating the
deadly high-THC varieties to low-THC plants. Rothschild noted, "...should these compounds [cannabinoids] exert a
fatal fascination for tiger caterpillars, it suggests another subtle system of insect control by plants."

DiscussionResearch demonstrates the moderate efficacy of Cannabis as a repellent crop or botanical pesticide. We are
faced, however, with a paradox: some of the pests controlled by Cannabis are also known to attack hemp and
marijuana crops. If Cannabis serves as an effective pesticide, then how can the pests infesting Cannabis survive?
Perhaps they intersperse marijuana meals with less-toxic lunches on other plants. Caterpillars of
Spilosoma obliqua, for instance, eat Cannabis leaves and female flowers (Nair
and Ponnappa 1974). But when Deshmukh et al. (1979) force-fed S. obliqua caterpillars a pure Cannabis diet, they
died after 20 days.Obviously Cannabis is not a cure-all poison; a highly
toxic botanical insecticide could not also serve as a human medicament, as Cannabis does (Clarke and Pate 1994,
McPartland and Pruitt 1997). Indeed, plants that are never infested by insects are considered dangerous by
traditional Ayurvedic healers in India (Thatte et al. 1993). What active ingredients in Cannabis work so well against pests?
Is THC the primary pesticidal ingredient? Characterizing THC as a powerful pesticide would put it in the same
category as nicotine, which is highly toxic to arthropods, fish, birds, and mammals. Although THC and other
cannabinoids have proven activity against bacteria and fungi, their activity against insects is questionable. The study by
Rothschild et al. (1977) assumed the sole difference between high-THC and low-THC plants was the level of
THC. Recent work has shown that high-THC plants produce 3 to 6 times more limonene and pinenes than most
low-THC plants (Mediavilla and Steinemann 1997). Cannabinoids are very safe to mammals. The oral LD
50 of THC in mice is greater than 21,600 mg/kg (Loewe 1946), safer than neem oil.Cannabinoids probably play a small role in
Cannabis’ pesticidal activity, but aqueous
Cannabis extracts work very well as pesticides, even if they contain
little cannabinoid. Cannabis contains more than 400 chemicals (Turner et al. 1980). Their leaf glands ooze
dozens of volatile compounds, such as terpenes, ketones, and esters which produce the characteristic odor of the
plant (Ross and ElSohly 1996). The limonene and several pinenes which comprise over 75% of volatiles detected in
the "headspace" atmosphere surrounding Cannabis leaves (Hood et al. 1973, Ross and ElSohly 1996) are powerful
insect repellents. Methyl ketones present in Cannabis
(Turner et al. 1980) also repel many leaf-eating insects
(Kashyap et al. 1991). A synergistic combination of these many compounds may serve as the "active ingredient" inCannabis.